The present invention relates to an image displaying apparatus for displaying images with digitally limited intermediate gradation levels, such as, plasma display panels (PDP), field emission display (FED)s, digital micro mirror devices (DMD) and electroluminescence displays (EL), and video signal processor. This invention also relates to video signal processing and error diffusion (variance) processing used for such image displaying apparatus.
Among image displaying apparatus, matrix-type image displaying apparatus, such as, a PDP for displaying images with gradation by dividing one field into sub-fields, a FED for displaying images with gradation by pulse-width modulation (PWM), and also a DMD, display images only at digitally-limited intermediate gradation levels.
Television broadcast generally provides gamma correction at the transmission side for cathode-ray tubes (CRTs) as receivers to achieve the linear characteristics with the reverse-gamma characteristics of CRTs at the reception side.
Contrary to this, image displaying apparatus that display images at digitally-limited intermediate gradation levels as listed above them-selves have the linear characteristics. Such image displaying apparatus thus require reverse-gamma correction to the 2.2 power to input video signals to regain the linear characteristics for displaying images with the gradation characteristics similar to that for CRTs.
These image displaying apparatus may receive input signals at gradation levels, the number of which (the number of bits) is larger than that can be displayed by the image displaying apparatus. These image displaying apparatus may decrease the number of gradation levels (the number of bits) to be displayed so that it is smaller than the number of gradation levels (the number of bits) for input signals. Moreover, the image displaying apparatus may increase the number of bits so that it is larger than that the displaying apparatus are capable of displaying when regaining the linear characteristics by reverse-gamma correction.
Decrease in the number of bits (gradation levels) is thus required when the number of bits (the first number of gradation levels) of input video signals or reverse-gamma-corrected video signals is larger than that (the second number of gradation levels) the image displaying apparatus being capable of displaying. Such a decrease however loses gradation; hence multi-gradation processing with error diffusion is required.
Multi-gradation processing by way of error diffusion is performed as discussed below with reference to FIG. 1 for displaying images corresponding to the first number of gradation levels larger than the second number of gradation levels explained above.
A dot “P” in FIG. 1 is one of the three dots for constituting a pixel, and has gradation that cannot be expressed at the second number of gradation levels. Dots “A”, “B”, “C” and “D” are located at the right side of, the left lower side of, under, and the right lower side of the dot “P”, respectively.
Multi-gradation processing is generally performed for offering images at quasi-first number of gradation levels by diffusing the gradation levels that corresponds to (the first number of gradation levels—the second number of gradation levels) to the surrounding dots “A” to “D” with a predetermined weighting for the dot “P” that can not be exhibited at the first number of gradation levels.
Displaying images at gradation levels by using the upper 8 bits of 12 bits of dot data by image displaying apparatus capable of displaying images at 8-bit gradation levels is performed by diffusing the dot data of the lower 4 bits of the 12 bits with constant weighting to the surrounding dots “A” to “D” for visual integration to achieve image displaying at gradation levels corresponding to 12 bits.
In FIG. 1, “7/16”, “3/16”, “5/16” and “1/16” are examples of error diffusion coefficients that represent the degree of weighting. The same error diffusion coefficients are used for the three primary colors R(red), G(green) and B(blue).
Conventional image displaying apparatus as introduced above have the following drawbacks:
Displaying images with reverse-gamma correction to digitally-converted R-, G- and B-video signals, as disclosed above, loses gradation particularly in the low intensity range, thus resulting in discontinuous gradation levels.
Especially, PDPs produce gradation levels by selecting sub-fields with different weighting for luminescence that constitute one field. This causes large differences in intensity among gradation levels, to produce large variation in visual color saturation between close gradation levels in the low intensity range for single color and mixed colors. This results in color pseudo-contour disturbances in image quality or in images with color saturation in which some dot bits have been dropped, particularly, for dark images at flat color saturation levels.
To avoid such phenomena, multi-gradation processing, such as, using dither matrices or error diffusion achieves smooth color saturation in the low intensity range close to black. This processing, however, causes disturbances in image quality, like periodical color pattern noises on black dark images.
Moreover, in order to avoid such phenomena, Japanese Unexamined Patent Publication No. 1997(9)-198000 discloses a method of decreasing color saturation in the low intensity range. This method, however, uniformly decreases color saturation in the low intensity range even for images of a large saturation in the low intensity range, thus producing strange images.
Such drawbacks are revealed not only in the low intensity range, but also partially in the intermediate and high gradation ranges, depending on sub-field selection.
Error diffusion as illustrated in FIG. 1 uses the same error diffusion coefficients to the three primary colors R, G and B, thus causes image disturbances, such as, periodical pattern noises peculiar to error diffusion, especially, for displaying fixed patterns.
Displaying digital video signals generated by personal computers and fixed patterns on PDPs is performed with no error diffusion to avoid lowering in image quality image which would otherwise occur due to disturbances, such as, periodical pattern noises peculiar to error diffusion.
However, conventional displaying apparatus use the same reverse-gamma correction characteristics for multi-gradation processing in spite of error diffusion, or an optimum reverse-gamma correction characteristics for image displaying in combination of reverse-gamma correction and error diffusion. Displaying images without error diffusion thus remarkably loses gradation levels in the low gradation range, thus producing images with remarkably discontinuous gradation levels in which digital bits have been dropped.